1. Field of the Invention
The present invention relates to a process for producing an SOI substrate which has an excellent monocrystalline layer like a monocrystalline wafer on an insulating material with good uniformity and controllability in production.
2. Related Background Art
Formation of a monocrystalline semiconductor layer on an insulating material is well known as a silicon-on-insulator technique (SOI). Many studies have been made of the SOI technique because of many advantages which cannot been achieved by a usual bulk silicon substrate for silicon integrated circuits.
One example of recently reported SOIs is a so-called "laminated SOI". This laminated SOI is produced by contact-bonding of two wafers with the respective mirror-polished faces having an insulation film on at least one of the faces, heat-treatment to strengthen the interface bonding, and polishing or etching of one of the wafers to leave a silicon monocrystalline thin film of a desired thickness (an active layer) on an insulating film. In this technique of laminated SOI formation, the most important step is polishing or etching of the silicon substrate into a thin film, in which the silicon substrate having a thickness of several hundred microns is polished or etched uniformly to a thickness of several microns or less than one micron. This process is extremely difficult technically in control and uniformity.
The processes of thin silicon film formation are classified into two types. In the one type of process, the thin film is formed by polishing only (BPSOI: Bonding and Polishing SOI). In the other type of process, an etch-stopping layer is provided immediately on the thin film to be left, and etching is conducted in two steps comprising etching of the substrate and etching of the etch-stopping layer (BESOI: Bond and Etchback SOI).
In the BPSOI processes, the thickness of the silicon film meant to remain cannot readily be controlled, because the polishing operation is conducted by reference to the face of the stand of the polishing machine on which the specimen to be polished is placed, and any thickness distribution of the supporting substrate wafer itself affects the thickness distribution of the active layer. Usually the thickness of the active layer of the thin film SOI should be controlled within several ten angstroms. However, the substrate itself cannot be controlled practically to have the thickness within such a range, and therefore the BPSOI process cannot readily control the thickness of the active layer.
In most of the BESOI processes, on the other hand, the active silicon layer is formed by epitaxial growth on a preliminarily formed etch-stopping layer. Therefore, at the moment, the BESOI processes are considered to be advantageous in achieving uniformity of film thickness. However, the etch-stopping layer frequently contains an impurity at a higher concentration, and the selectivity of the etching in most cases depends on the impurity concentration. This means that diffusion of the impurities in annealing or like heat treatment after the lamination can change the etching characteristics. Generally, the etching rate, or the etching selectivity, changes depending on the impurity concentration at a ratio of several tens or several hundreds at the highest, which necessarily causes excessive etching or thickness distribution of the active layer.
A method is proposed by the inventors of the present invention in which the active layer has uniform thickness as in BESOI, and the selectivity of the etch-back is many-fold higher by a factor of several digits than the conventional BESOI. In the proposed method, the surface of the silicon substrate is made porous by anodization, an active silicon layer is epitaxially thereon, and the formed silicon layer is transferred onto another substrate by lamination to prepare an SOI substrate (Japanese Patent Application Laid-Open No. 5-21338). This method is based on the extremely high etching rate of the porous silicon by an etchant in comparison with that of the non-porous monocrystalline silicon, and enables production of an excellent SOI substrate at a low cost.
An etchant applicable to the aforementioned process has been proposed also by the applicants of the present invention (Japanese Patent Application Laid-Open No. 6-342784, etc.). According to the studies made by the inventors of the present invention, the high etching selectivity of the porous silicon can be obtained by use of a hydrofluoric acid type etchant such as a mixture of hydrofluoric acid and hydrogen peroxide.
However, it is reported recently that the hydrofluoric acid type etchant also etches crystal defects in the active layer (Proceedings of IEEE International SOI Conference, 1994, p.111). As the result of investigations made by the inventors of the present invention, this action of the etchant may form pinholes in the active layer, may corrode the underlying oxide film through the pinholes, or may form voids in the lamination interface between the substrates.